2. Field of the Invention
The present invention relates to a catalytic cracking process and apparatus, particularly a fluid catalytic cracking unit ("FCCU").
2. Description of the Prior Art
In contemporary catalytic cracking processes, the feedstock is contacted with particles of hot, active cracking catalyst at a suitably elevated temperature whereby the feedstock is at least partly converted to vaporous cracked products in endothermic reactions. The products are separated from the resultingly cooled used catalyst and recovered, and the cooled used catalyst is separately recovered. The used catalyst is associated with hydrocarbon material which is disposed in the spaces between catalyst particles and also adsorbed in and on the surfaces and pores of the particles. The used catalyst particles and associated hydrocarbon material are subjected to a stripping process to remove from the particles as much hydrocarbon material as is technically and economically possible, the thus-removed hydrocarbon material is recovered. The stripped particles and remaining associated hydrocarbon materials are passed to a regenerator wherein the remaining associated hydrocarbon materials are removed from catalyst particles by oxidation with an oxygen-containing gas. The oxidation reactions are strongly exothermic and the resulting regenerated catalyst particles of substantially reduced hydrocarbon material content are thereby heated to an elevated temperature at which they can be used for contacting further quantities of feedstock.
There are technical and commercial incentives to ensure that the stripping process is as effective as possible. From the technical viewpoint, the oxygen-requirement for the regeneration step is increased for increases in the amount of hydrocarbon material associated with catalyst material undergoing regeneration. The amount of oxygen-containing gas required for regeneration determines the size of the regeneration equipment, including the blower for the oxygen-containing gas, the regenerator vessel, the gas ducting, and regenerator overhead gas treatment facilities, and thereby the capital cost of the foregoing. Moreover, an increase in oxygen requirement necessitates the use of a higher-capacity blower which, in turn, requires more power for its operation, thereby adding to the increased costs of the plant. Furthermore, the oxidation of relatively large amounts of hydrocarbon material generates heat which, if excessive, can damage the catalyst particles and also the regenerator equipment.
From the commercial viewpoint, the oxidation of hydrocarbon material in the regenerator represents a loss of hydrocarbon material which might otherwise add to the products obtained in the catalytic cracking process. Furthermore, for existing FCCU of limited coke burning capacity, a reduction in strippable hydrocarbon entering the regenerator would permit an increase in other coke-making factors e.g. reactor intensity, feed rate or feed quality, hence increasing FCCU profitability.
There are therefore incentives to separate from used catalyst particles as much hydrocarbon material as possible. Such separation is often designated "stripping" and will be so referred to herein, from time-to-time.
One way in which the effectiveness of stripping can be enhanced is by raising the temperature at which the stripping is performed.
U.S. Pat. No. 4,789,458 discloses a typical fluid catalytic cracking process with the added concept of recirculation of hot regenerated catalyst to heat the spent catalyst in the stripping zone to thereby improve stripping of hydrocarbon volatiles on the spent catalyst. This patent does not disclose contacting the hot regenerated catalyst stream with a hydrocarbon stream prior to entering the stripping zone.
U.S. Pat. No. 2,451,619 also discloses the broad concept of using regenerated catalyst particles to heat spent catalyst in the stripping zone. This patent discloses that the hot regenerated catalyst to be introduced into the stripping chamber may be maintained in an aerated liquid-like condition by means of aeration gas introduced with the regenerated catalyst circulated to the stripping zone. Aeration gas may be steam or an inert gas or it may be a hydrocarbon gas. In the event a hydrocarbon gas is used as the aeration gas its purpose is to affect the displacement of any oxygen containing gases by burning the remaining oxygen.